Thought I'd better add an update here. Turns out my "clever" method wasn't so clever after all.
It works fine for just generating index pages with content matching the actual site, and it's also fine for duplicating all permissions, etc from the actual site. That's largely what I was after initially. However, there are functionality glitches in some places, like when clicking on a link to a topic, which is problematic when testing various customisation tweaks. So, what I ended up doing was importing the whole database to local.
To get around the limits of phpMyAdmin config I bypassed it entirely, and imported the db direct via MySQL on the command line. Turns out you can do this quite easily on a Windows desktop (basic instructions here: https://john-dugan.com/dump-and-restore ... n-windows/
). I found this to be the simplest method. The entire 140 meg db imported in seconds.
It was then simply a matter of dropping all content (but not structure, of course) from the private messages tables in phpMyAdmin (because you have no business having people's PM's on your test site) and then pruning a lot of members and old topics to knock things down to a useful amount (any existing account can have password and email reset to handy test values).
This works. Everywhere.